Boil, peel and roughly mash the potatoes. Pour the hot milk over it, add the butter and stir vigorously with the whisk or in the food processor - there may still be small pieces in it. Squeeze the garlic clove and add, season with salt, pepper and nutmeg. If necessary, add a little milk, the porridge should be relatively soft.
Thaw the spinach according to the package instructions and bring to the boil, stir in the cream and season well with salt, pepper and nutmeg.
Rinse and pat dry the fish fillets, cut into bite-sized pieces. Season with lemon juice, salt and pepper.
Grease a large baking dish and pour in the hot spinach. Spread the pieces of fish on top and spread the mashed potatoes over them. Finally, sprinkle with the grated cheese.
Bake in the oven at 175 ° C (fan oven) for about 30 minutes until the cheese is lightly browned.